The development of tumor cell resistance to natural product anti-cancer drugs is widely recognized as a major limiting factor in patient responsiveness to chemotherapy programs. A common mechanism for the acquisition of tumor cell multi-drug resistance (MDR) is through increased activity of P-glycoprotein, which acts as an energy-dependent drug efflux pump. It is therefore likely that the identification of compounds which circumvent this mechanism of resistance will enhance the successfulness of cancer chemotherapy. We have established a program to identify such anti-MDR compounds. In a collection of extracts from different species of cyanobacteria, two classes of compounds which circumvent P-glycoprotein-mediated MDR have been identified. Firstly, certain members of a family of macrolides called scytophycins have been found to demonstrate equivalent cytotoxicities toward normal cells and MDR cells, i.e. they do not appear to be transported by P-glycoprotein. Secondly, a family of novel porphyrins which chemosensitize P-MDR cells has been isolated. We have also recently found that closely structurally related cytochalasins interact very differently with P-glycoprotein. Studies conducted under the present proposal will characterize the biochemical and pharmacological anti-MDR properties of these natural products. The effects of each chemosensitizing compound on P-glycoprotein activity (drug binding and transport), P-glycoprotein expression (both protein and mRNA) and MDR cell chemosensitivity will be established. Cellular responses to longterm exposure to these agents will be examined. Additional active compounds in these extracts are currently being isolated, and their pharmacological properties will be similarly characterized. These studies may lead to the identification of new compounds which either inhibit P-glycoprotein activity or which elude the Pglycoprotein mechanism of resistance. Such compounds may allow new approaches to the chemotherapy of drug-resistant cancers.